Crosstalk between PKA and MAPK signalling during mating in Ustilago maydis
Final Report Abstract
Pheromone signalling during mating is essential for pathogenicity of Ustilago maydis. One of the key regulators is the transcription factor Prfl orchestrating the underlying transcriptional programme. Its activity is controlled at the transcriptional level as well as posttranslationally via MAPK and PKA phosphorylation. The aim of this project was to elucidate the molecular consequences of PKA as well as MAPK regulation of Prfl activity. Therefore, we combined genetics with transcriptional profiling to investigate regulation of this central signalling node. Uncoupling transcriptional activation by constitutive prfl expression hardly influenced the pheromone response of target genes stressing the importance of posttranslational control. Gain-of-function mutations at PKA sites of Prfl triggered regulation of a small group of nine pheromone-responsive genes including the key transcription factor for pathogenic development. Loss-of-tunction mutations at MAPK sites altered the pheromone response of a defined subset of 57 genes. These exhibited two distinct expression patterns with abolished or reduced pheromone regulation. Thus, MAPK regulation of Prfl is important for either qualitative or quantitative control of pheromone-responsive expression. Since a substantial portion of the quantitative control could be mimicked by overexpression or by transcriptional prfl activation via sustained MAPK signalling, this mode of regulation most likely depends on the amount of Prfl. Expression of genes underlying qualitative control could not be triggered by pheromone-independent MAPK activation emphasising that crosstalk with a second signalling branch is essential. These results indicated a novel level of complexity in MAPK signalling suggesting that target genes can respond differentially to MAPK phoshporylation of the respective transcription factors. In essence, we discovered that PKA phosphorylation of Prfl is most likely involved in regulation of a small subset of pheromone-responsive genes including the mating-type genes, whereas MAPK phosphorylation of Prfl is used to either qualitatively or quantitatively control the expression of a distinct subset of pheromone-responsive genes. These results suggested that the fungus integrates environmental signalling via the cAMP pathway with pheromone signalling via the MAPK module at the level of transcription factor phosphorylation to orchestrate mating.
Publications
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(2006) Tetracycline-regulated gene expression in the pathogen Ustilago maydis. Fungal Genet. Biol., 43,727-738
Zarnack, K., Maurer, S., Kaffarnik, F., Ladendorf, O., Brachmann, A., Kämper, J, and Feldbrügge, M.
